The present invention relates to a laser beam scanner, and more particularly to a laser beam scanner using a hologram for light deflection.
A holographic laser beam scanner utilizes a hologram as a diffraction grating, and achieves light deflection by moving a hologram across an incident laser beam. The hologram has a variable pitch grating which is adapted to vary and thereby varying the diffraction angle of the laser beam. A number of proposals have already been made as to a hologram for use in such a laser beam scanner.
The present inventor devised a hologram which was considerably free of astigmatism, which constituted a problem with prior are holograms, and acquired the U.S. Pat. No. 4,299,437 on a laser beam scanner using that hologram. That invention used a hologram which was produced by interference between a divergent spherical wave and a convergent spherical wave and had on it a phase distribution .phi.(r) of interfering light beams represented by the following equation. ##EQU1## In a disk type laser beam scanner a hologram disk, on whose periphery is arranged such a hologram, is rotated to move the hologram across an incident laser beam. In such scanner it is advantageous to increase the locus length of the scanning laser beam on the scanned surface relative to the locus length of the incident laser beam on the disk, i.e. the scanning magnification. To this end the diffraction angle .theta..sub.d of the hologram is designed to be as great as practicable. The calculations of the scanning magnification, will be explained below with reference to FIG. 1. In the figure, an incident laser beam 32 strikes a hologram disk 33 and is diffracted as a scanning laser beam 35, which draws a line 36 as the hologram disk 33 rotates. The rotational radius R.sub.F of the scanning beam on the focal plane containing the focus 37 of the hologram, separated from the hologram by the focal distance F of the hologram, is represented by Equation (2) below: EQU R.sub.F =F.multidot.tan .theta..sub.d +R (2)
where .theta..sub.d is the diffraction angle of the scanning beam when it comes to the center of the scanning line, and R, the disk radius in the striking position of the incident beam 32. The scanning length l.sub.p on the scanning surface is represented by Equation (3) below: EQU l.sub.p =R.sub.F .multidot..theta..sub.r .multidot.L/F (3)
where L is the distance between the hologram and the scanning surface and .theta..sub.r is the rotational angle of the disk. Meanwhile, since the locus length h of the incident beam on the hologram when the disk has rotated by .theta..sub.r is EQU h=R.multidot..theta..sub.r ( 4)
the scanning magnification M is represented by ##EQU2## on the basis of Equations (2), (3) and (4). Equation (5) reveals that as the diffraction angle .theta..sub.d becomes larger, the the scanning magnification becomes larger. Thus, if the diffraction angle .theta..sub.d is set greater, a sufficiently long scanning line can be obtained even though the rotational angle of the disk is small. Therefore, it is advantageous to set the diffraction angle .theta..sub.d greater in designing the hologram disk. However, for the hologram disclosed in the aforementioned U.S. Pat. No. 4,299,437, though the beam is imaged on the scanned surface within the diffraction angle range of 0.degree. to 30.degree., it is subject to a conspicuous astigmatism when the diffraction angle .theta..sub.d is greater than 30.degree.. If the diffraction angle .theta..sub.d is limited to this maximum angle, there is a corresponding restriction on the scanning length of the light beam scanner.